The invention is based on a method for manufacturing steel housings, composed of at least two housing components, for assemblies installed in vehicles, in particular in utility vehicles.
In order to activate brakes in utility vehicles, use is made, inter alia, of combined pneumatic service brake cylinders and spring brake cylinders. Such a combination cylinder is, connected in series, a combination of a service brake cylinder for activating the service brake and a spring brake cylinder for activating the auxiliary (parking) and spring brake.
A method is known from EP 1 136 337 B1, which discloses a combined service brake cylinder and spring brake cylinder of a utility vehicle, in which the housing of the service brake cylinder and the housing of the spring brake cylinder are connected to one another by the edge of the spring brake cylinder overlapping the edge of the service brake cylinder in a positively locking fashion, which is also referred to as beading.
Such beading is frequently produced by rolling, i.e. the edge of at least one housing component is shaped plastically by a circulating rolling tool. However, at least the outwardly pointing surface of the housing components must firstly be provided with a corrosion prevention means since assemblies of vehicles, such as, for example, combined service brake cylinders and spring brake cylinders of utility vehicles which are arranged near to the wheel, are subjected to moisture which is permeated by de-icing salt. Corrosion prevention layers which are used in this context are composed, for example, of a sprayed-on epoxy plastic layer or of a simple zinc layer. However, it has become apparent that the corrosion prevention layer can be damaged by the beading fabrication step and therefore the necessary resistance to corrosion is not achieved, the resistance being, for example, that the housing has to withstand over 400 hours of uninterrupted wetting with salt water without corroding.
The invention is based on the object of developing a method for manufacturing housings, composed of at least two housing components, for assemblies installed in vehicles, in particular in utility vehicles, in such a way that it permits beading which shapes the housing components to be fabricated without adversely affecting the corrosion prevention means of such housing components.
This object is achieved according to the invention by application of a corrosion prevention layer comprising zinc or an alloy containing zinc to at least one outwardly pointing surface of at least one housing component with subsequent passivation, and manufacture of a positively locking overlap by the edge of one housing component over the edge of the other housing component by way of relative movement of the housing components which have previously been oriented with respect to one another and in relation to a die which plastically shapes the edge of one housing component and/or the edge of the other housing component.
The invention is based on an advantageous combination of two method steps, specifically on the one hand the fabrication of beading by use of a die and, on the other hand, the coating of the surface with a corrosion prevention layer comprising zinc or an alloy containing zinc, and subsequent passivation. It has become apparent that such a corrosion prevention layer is not damaged if the beading is manufactured with a die.
The protection against corrosion which is provided by the zinc or the other metal alloys is due to the fact that it is even more stainless than the basic metal steel and therefore functions as the sacrificial anode. As long as the covering zinc layer is present, the basic metal is protected against corrosion. Other metals can influence the corrosion protection provided by the zinc layer. Iron, nickel or cobalt are therefore used selectively as alloy components for the purpose of bringing about improvement. The zinc is itself protected against corrosive attack by the passivation, generally by chromatization. The thicker, denser and more chemically resistant the layer of chromate, the better the barrier effect.
As a result of this, a high degree of resistance to corrosion is provided for a housing which is manufactured according to the inventive method, and a high degree of operational reliability is provided, which is essential in particular if the component is a brake cylinder.
Advantageous developments and improvements of the invention are described herein.
The assembly whose housing is fabricated by the method according to the invention is preferably a brake cylinder, in particular a combined service brake and spring brake cylinder, an air dryer or a brake booster of a utility vehicle, therefore assemblies with sheet metal housings which have at least two components and are, for example, deep drawn.
The relative movement between the die and the housing components particularly preferably occurs in an axial direction with respect to a central axis of the assembly. Alternatively, the relative movement can occur in a radial direction with respect to the central axis of the assembly.
The die can be embodied as a single-component or multi-component ring with a radially inner recess which exerts pressure against the edge of the one housing component and/or the edge of the other housing component. In the case of an axial relative movement between the die and the housing halves, the die can be embodied as a single component or multiple components, and in the case of a radial relative movement it can only be in multiple components, for example if the housing components have regions which project radially beyond their end edges and would therefore collide with a single-component ring which is guided in the axial direction and surrounds the housing components.
Before the application of the die, for example one housing component is clamped in through a clamping device and the other housing component is fitted onto the latter in such a way that the edges of the housing components overlap. The reaction forces which arise during the deformation are held then by the housing components themselves or by structures which are located in the interior, and are conducted away via the clamping device. In cases in which this is possible, a die plate which is assigned to the die can be dispensed with, otherwise, if the housing components are not sufficiently rigid, there has to be a fixed die plate which absorbs the reaction forces and can additionally serve to shape the beading by being provided with corresponding recesses itself.
In order to keep the forces occurring during deformation low, before the application of the die, the edge of one housing component can be preshaped to form a shoulder which projects radially outward. This may have already taken place within the scope of a preceding deep drawing process. Particularly good rigidity of the beading connection is obtained if this shoulder has an undercut cross section.
Particularly good protection against corrosion is obtained if the corrosion prevention layer has pure zinc, a zinc/iron alloy, or an alkaline zinc/nickel alloy, and the passivation includes chromatization. In order to meet the EU guideline 2000/53/CE, solutions containing chromium (VI) (chromium in the hexavalent state) should be avoided.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.